This invention relates to a constant speed control device for a vehicle and, particularly, to a constant speed control device provided in an engine having a supercharger.
There is known an engine of a kind in which an amount of fuel in proportion to an amount of air supplied to the engine is injected from an injector into a combustion chamber of the engine, the latter being provided with a supercharger and a constant speed driving device. In constant speed driving devices of this kind, the opening of a throttle valve is controlled by an actuator operated by negative pressure from an intake manifold of the engine and controlled by a control signal from a computer or central processing unit, so that a constant driving speed is established.
In such an engine, when the supercharger operates, the negative pressure in the engine intake manifold tends to be reduced. This tendency becomes enhanced as the engine load increases, when the vehicle is driving on an upward slope, for example. The actuator operated by such reduced negative pressure insufficiently operates and controls the opening of the throttle valve so as to make desired constant speed driving impossible. To solve this problem, a system has previously been adopted in which an electrically operable vacuum pump is additionally provided and operated to increase the negative operating pressure. However, the provision of the vacuum pump renders the entire construction complicated and expensive.